The process of the present invention relates to methods of producing syndiotactic vinyl aromatic polymers.
In the production of syndiotactic vinyl aromatic polymers such as syndiotactic polystyrene (SPS), a devolatilization step is typically used to remove residual monomers, process solvents, and other volatile components from the SPS polymer. This process is complicated by the fact that residual vinyl aromatic and other monomers can autopolymerize upon heating to form atactic vinyl aromatic and other polymers, for example, atactic polystyrene, which are unwanted contaminants in SPS polymers. Atactic vinyl aromatic polymers degrade the SPS polymer properties such as heat distortion temperature and reduce the crystallization rate of SPS homopolymer and copolymer resins.
In order to prevent discoloration of the SPS polymer, the devolatilization process is typically preceded by a deashing step to extract active catalyst residues. Deashing requires treatment of the polymer with a deashing agent such as hydrochloric acid, potassium hydroxide and the like. Alternatively, at low catalyst levels, active catalyst residues can simply be deactivated prior to devolatilization, thereby remaining in the final resin. Deactivation is typically achieved by intimate mixing of the polymer with an active nucleophilic agent, preferably a protic solvent such as methanol.
Several methods of devolatilization are known in the art, including melt devolatilization wherein the polymer is first melted and then devolatilized in the fluid state; and solid state devolatilization, wherein solid polymer is heated and devolatilized at a temperature between the glass transition temperature and the melting point of the polymer.
JP 03056504 by Yamamoto discloses a melt devolatilization process, wherein wet SPS powder containing volatiles is fed to a twin screw extruder where it is melted and devolatilized. Although volatile residues are reduced, a catalyst deactivation or deashing step is needed to prevent discoloration due to the presence of active catalyst residues.
JP 03064303 by Yamamoto discloses a two step solid state devolatilization process, wherein wet SPS powder containing volatiles is first fed to a dryer where it is heated to a temperature between the glass transition temperature and the melting point of SPS and further devolatilized by melt devolatilization in a vacuum vented twin screw extruder as described in JP 03056504, above. However, this method is very time consuming, taking 9 or 10 hours to complete, and a catalyst deactivation or deashing step is also needed.
Several methods of deashing and deactivation of active catalyst residues from SPS polymers are known. U.S. Pat. No. 5,321,122 issued to Kuramoto et al. discloses a process of purifying a styrene polymer by deashing with an alcoholic alkaline solution and washing with an alcohol. U.S. Pat. No. 5,426,176 issued to Teshima et al. discloses a process for purifying a styrene polymer by deashing with a deashing agent, for example HCl, KOH, at a temperature which is greater than or equal to the glass transition temperature of the polymer. U.S. Pat. No. 5,449,746 issued to Teshima discloses a method of purifying a styrene polymer by treating with a swelling agent, for example ethylbenzene, and a deactivating agent, for example methanol or ethanol. U.S. Pat. No. 5,612,452 issued to Teshima and Yamasaki discloses a process for simultaneously deactivating and deashing crystalline styrene polymers by treating the polymers with a poor solvent containing 15 to 10,000 ppm water. However, these methods are additional finishing steps which increase the manufacturing complexity and cost of the SPS polymer.
Therefore, there remains a need for a process of devolatilizing syndiotactic vinyl aromatic polymers which does not involve the additional operation of deashing or deactivating catalyst residues while producing polymers having reduced volatiles, reduced discoloration and improved whiteness.